Titanium dioxide is, in its pure form, a valuable white pigment used in paints. Traditionally, the pigment has been obtained using the "Chloride" or "Sulphate" processing routes. The "Sulphate" processing route, which has fallen into disfavour in recent times due to the environmental problems caused by the disposal of acidic ferrous sulphate liquors, involves the pug-roasting of ilmenite with concentrated sulphuric acid to form a solid sulphate cake which is then dissolved in hot dilute sulphuric acid to liberate iron. Ferric ion is removed from the solution by reduction with scrap iron and ferrous ions are partially removed from the solution by crystallisation of hydrated ferrous sulphate (copperas). Hydrous titanium dioxide is calcined and further processed to produce the pigment.
In order to overcome the environmental problems inherent in the "Sulphate" route, the "Chloride" route has become commonplace. The "Chloride" process involves the chlorination of natural or synthetic rutile at high temperatures to form a mixture of titanium tetrachloride and ferric chloride vapours which are separated from each other by selective condensation and, if necessary, further refining steps. The titanium tetrachloride is then oxidised to produce pigment grade titanium dioxide or reduced to produce titanium metal in accordance with the Kroll process.
It is clear that in both of the above processing routes the removal of iron is a problem, which at the very least involves further capital costs to address. For example, ferric chloride may be oxidised in a costly further processing stage to iron oxides and the chlorine recovered for further use in a chloride process.
Moreover, as the world reserves of natural rutile, containing approximately 95% by weight titanium dioxide fall, the iron removal problem becomes more acute. Therefore, alternative processes have been devised for the environmentally acceptable production of synthetic rutile from the abundant available reserves of ilmenite, a mixed iron and titanium oxide ore, represented approximately by the formula FeTiO.sub.3. Ilmenite contains about 53% by weight titanium dioxide.
The majority of such processes include hydrometallurgical steps but generally commence with partial or complete reduction of the iron oxides present within the ilmenite to metallic iron by pyrometallurgical processing followed by selective leaching of the iron with hydrochloric or sulphuric acids, ferric chloride or aerated water. The process where aerated water is used is called the Becher Process, and is essentially an accelerated iron "rusting" process wherein, commonly, a salt such as ammonium chloride is also added as a corrosion promoter.
The process commences with a carbothermic reduction step in a rotary kiln at 1150.degree. C. to which coal or a suitable carbonaceous agent is added, both as a reductant and as a fuel. The reduced ilmenite from the kiln is then cooled and further treated with an aerated aqueous solution, ideally containing 1% by weight ammonium chloride, in a stirred batch reactor in order to rust out the iron over a period of 12 to 16 hours.
The ammonium chloride is believed to give benefits in terms of an accelerated iron corrosion rate for two major reasons. Firstly, the chloride ions can break down passive oxide filme which may have formed on the metallic iron during the reduction stage while, secondly, the ammonium ion acts as a buffer against very high pH values near the surface of the reacting iron. This assists in the removal of hydroxyl ions by the reaction: EQU NH.sub.4 +(.sub.aq)+OH--.fwdarw.NH.sub.3 +H.sub.2 O (I)
If the hydroxyl ions were not removed by this reaction they would react with ferrous ions produced by the half reaction: EQU Fe.fwdarw.Fe.sup.2+ +2e.sup.- (II)
to produce a precipitate of hydrated ferrous oxide in the pores of the ilmenite, thus slowing and possibly completely stopping the rusting reaction by reducing or eliminating the supply of oxygen to the metallic iron within the ilmenite grains. This is a limitation of the conventional process. Upon completion of the reaction, the iron oxides produced are removed from the reaction mixture by washing in hydrocyclones. The synthetic rutile is then subjected to a final hydrochloric acid leaching step to remove residual iron and produce a product grading 92 weight % titanium dioxide which is suitable as a feedstock to pigment or metal production processes.
A clear disadvantage of the above process, however, is that it is rate-limited by the diffusion rate of oxygen to the metallic iron which comes about due to the low solubility of oxygen in aqueous solutions.